A U.S. counterterrorism official said the warning was intended to boost awareness of the threat of al Qaeda attacks such as the Cole, which killed 17 U.S. sailors, and a later attack on a French oil tanker that killed one crew member.

"Extremists on the Arabian Peninsula continue to look to maritime interests as possible terrorist targets," the official said.

According to the warning notice, the exact method of any planned attack is not known but it "may be similar in nature to the attacks against the USS Cole in October 2000 and the M/V Limburg in October 2002, where a small to mid-size boat laden with explosives was detonated."

More sophisticated attack methods could include missiles or projectiles fired at ships.

"Although the time and location of such an attack are unknown, ships in the Red Sea, Bab-al-Mandeb Strait, and the Gulf of Aden along the coast of Yemen are at the greatest risk of becoming targets of such an attack," the report said.

The danger zone for the attack is near Yemen, and all vessels in the region were urged to operate at a heightened state of security, including the use of 24-hour visual and radar watches.

"Vessels are at greatest risk in areas of restricted maneuverability, and while in [or] near port or at anchor," the notice said.

Merchant vessels were requested to report any suspicious activity to the United Kingdom Maritime Trade Operations office in Dubai.

The area covered by the warning has been one of the most active areas for Somalia-based pirate attacks, though targeting by Islamist extremists has been rare in recent years. According to government anti-piracy reports, there were 23 pirate attacks or suspected pirate incidents against ships in the Red Sea, Gulf of Aden, Bab-al-Mandeb Strait, and western Indian Ocean waters near Africa and the Middle East in the past two months.

Samir al-Abdali, an analyst at the Yemen Center for Research and Studies, said the increased al Qaeda presence in Yemen and Somalia is aimed at controlling and closing the strategic Red Sea, a major shipping route.

Mr. al-Abdali said the Red Sea straits, islands and strategic corridors are centers of a "tug of war" between the United States, Israel and the West on one side and al Qaeda on the other.

"Though I doubt the organization's ability to control the Red Sea, I still believe that is it a warning of the organization's ability and plans for the coming stage to deliver sporadic and painful blows to Western interests in the Red Sea, Bab-al-Mandeb Strait, and Gulf of Aden and to the Israeli bases in Eritrea, similar to what happened to USS Cole and the French destroyer Limburg," he told the London-based Internet daily news outlet Ilaf.com on Feb. 16.

A Navy spokesman referred questions to the Transportation Department. A spokesman for the department could not be reached.

The Office of Naval Intelligence warning followed the disclosure in The Washington Times on Jan. 8 that an al Qaeda Web site had announced that the group planned to launch a campaign of attacks against Navy interests in response to stepped-up U.S.-backed targeting of terrorists in Yemen.

Al Qaeda in the Arabian Peninsula, the Yemen-based affiliate of Osama bin Laden's terrorist group, announced Dec. 29 that it was calling on Muslims to wage a "mast media campaign of gathering information on U.S. Navy ships, their crews, how they are serviced by other nations, and data on possible nuclear weapons on board.

The statement said al Qaeda had targeted the Navy in the past by bombing the Cole and that "every American naval vessel in the seas and oceans: aircraft carriers, submarines, and all of its war machines within range of al Qaeda will be destroyed."

Navy officials at the time said the sea service had heightened its security in response to the threat.

Al Qaeda in the Arabian Peninsula also claimed credit for the attempted Christmas Day bombing over Detroit and cheered the Fort Hood, Texas, shootings that have been blamed on Army Maj. Nidal Malik Hasan, who had extensive e-mail contacts with Yemen-based Islamists.

The bombing of the USS Cole on Oct. 12, 2000, in the Yemeni port of Aden was carried out by a small boat laden with an estimated 700 pounds of explosives that approached the destroyer and detonated, killing 17 sailors and ripping a hole in the hull that nearly sank the ship. Al Qaeda claimed responsibility.

The Pentagon said after an investigation of the attack that the terrorists had exploited a gap in security involving local refueling in Yemen to get close to the ship.

The Navy since the Cole bombing has adopted what it calls a surface-warfare mission package designed to counter small-boat attacks. It includes electro-optical/infrared sensors on unmanned aerial vehicles; 30 mm guns to kill close-in targets; "missile-in-a-box" launchers and MH-60R armed helicopters.

A Defense Intelligence Agency analyst, Kie Fallis, resigned in protest after the bombing, saying his warnings that al Qaeda was planning a small-boat terrorist attack in the region were ignored.

The second al Qaeda attack in the region was the bombing on Oct. 5, 2002, of the French supertanker Limburg that was carrying nearly 400,000 barrels of crude oil to Malaysia when it was rammed by an explosives-laden dinghy.